Sound-reproducing systems



Oct. 25, 1955 v. A. FOOT SOUND-REPRODUCING SYSTEMS 2 Sheets-Sheet 1 Filed July 20, 1950 Invnlor United States Patent SOUND-REPRODUCING SYSTEMS Victor Albert Foot, London, England Application July 20, 1950, Serial No. 174,824

Claimspriority, applicationGreat Britain July 27, 1949 26 Claims. (Cl. 179-1) Thisinvention relates to sound-reproducing systems and is particularly though not exclusively'concerned with pub lic address systems of the kind in which individual members of an audience are .provided with headphones-either to increase :the volume of sound for the benefit of members of the audience "whose hearing is not good, or to provide sound differing from that normally audible, for example in a simultaneous translation system. The invention is also applicable to systems for use in hospitals and other places where the sound reproduction is to be available to those individuals wishing to listen, without disturbing others, or where for any reason ordinary loudspeaker reproduction is undesirable.

According to the present *inventiona sound-reproducing system includes in combination a coil installed in a building or other space-and energised from a sound-reproducing amplifier of suitable wattage and'frequencyoutput so as to produce a varying magnetic field throughout the space in which the sound is to .bepicked up, and one or more receiver 'units each including a magnetic core disposed so that a varying magnetic flux is produced in it by the coil, and anarmature responsive to the fluxinthe core and acting on a sound-reproducing diaphragm or the equivalent. In some cases the armature may be constituted by a part of the diaphragm. In one arrangement at least one receiver unit forms a portable unit which is freely movable by the user within the ":saidspace-and has noelectrical connection to other parts of the system.

Preferably the coil is arranged to produce flux having a substantial vertical component and the receivers are arranged to respond to such a component. In this way 'th'e sound reception can be substantially independent of the particular horizontal orientation of the receiver unit and the user can turn in 'various directions without losing the sound.

Alternatively in some cases it may be possible to transmittwo separate programmes so as to produce flux whereof the horizontal components are at right angles so that the listener LcanThear either at will by turning his head in the appropriate direction.

In 'one 'form of the invention the receiver unit, which maybe in'the form of a headphone, is arranged so that'the core is adjustable, conveniently about the axis of the diaphragm, so that it can be brought into or out of the line of maximum flux, thereby providing a convenient and simple form of volume control.

The magnetic circuit of the receiver unit may take various forms but preferably it comprises a pair of collecting horns or pole pieces of high permeability extending in opposite directions from the armature which proyides a path for the magnetic fiux between their-adjacent ends via small air gaps, and one or more permanent magnets arranged to produce a permanent flux through the air gaps.

The permanent magnet maybe connected between the pole pieces provided it is of low incremental permeability so that it will not by-pass or short-circuitthe changes of flux representing the sound to be reproduced. Alterand a pair of s'oft iron bridges J 2,721,896 Patented Oct. 25, 1955 ice natively a pair of permanent magnets may be employed which do not connect the two pole pieces, for example each pole piece may have a permanent magnet alongside it.

The overall length of the pole pieces may be such that they canbe accommodated in a casing of conventional size, for example of about the same diameter as the diaphragm. Alternatively they may be prolonged so as to project beyond the casing, or the latter may be formed with protuberances to accommodate them.

In one simple form of the invention the receiver does not include any energised coils acting on the core.

In an alternative form of the invention the receiver core is provided with coils connected and arranged to reinforce the flux picked up directly by the core. For example the coils of the receiver may be connected to a coil or coils wound on a core'which is also exposed to the flux in the space where the sound is to be picked up. The latter core may be a continuation of the core of the receiver unit, for example it maybe arranged in a handle attached to the receiver.

The invention may be carried into practice in various ways but'certain specific embodiments will be described by way of example with reference to the accompanying drawings, in which Figure l is a schematic diagram 'of the general scheme,

Figure 2 is a sectional side elevation of an earpiece of a receiver unit,

Figure 3 is a sectional front elevation of the earpiece,

Figure 4 is a fragmentary diagram showing a modified arrangement of the permanent magnet 'in the earpiece of Figures'Z and 3, and

Figure 5 is a sectional side elevation of a lorgnette type of receiver unit.

The invention is shown as applied to a public address system for ahall or building, in which case the sound to be reproduced is supplied'to an amplifier A, whether from a microphone, a radio receiver, a record reproducer, The output from the amplifier is able Wire, usually stranded or braided, installed around the walls or on the flo'or or ceiling of the building in such manner that when excited the coil will produce a magnetic field throughout that part of the building in which the sound reproduction is required.

Each individual listener who wishes to hear the sound reproduction is provided with a special pair of headphones C one earpiece C of which is shown in Figures 2 and 3.

In general appearance this earpiece may resemble that of a conventional pair of headphones, the main difference being that it :has "no coils. Thus the earpiece C comprises a casing D with a cover or cap E carrying a conical diaphragm F the centre of which is secured to a reed or armature 'G carried by the casing D. The reed orarma'ture G is presented to the gap between the adjacent ends of a pair of pole pieces H which extend in opposite directions from the armature and are composed of a material of 'high permeability. The pole pieces are bridged by apermanent magnetd which may either be of C-shape, or as shownin Figure 3 may be built up from a bar magnet Alternatively, as shown in Figure 4, the permanent magnet I may be letinto a recess in the pole pieces H so as to bridge them.

In operation the headphone is placed in the field of the coil B so that the length of the pole pieces is in line with the direction of the flux produced by the coil. The pole pieces collect and concentrate the fiux in their neighbourhood so that an appreciable flux passes through the air gaps and through the armature, thereby operating the diaphragm to reproduce the sound. The permanent magnet functions in the conventional manner to superimpose a steady flux to prevent reversal of the force exerted on the armature.

The cover of each headphone is connected to the usual headband, whilst the casing is connected to the cover in such a way that it can be turned about the axis of the diaphragm. Thus the margin of the cover is screwthreaded to receive a locking ring E which secures in place a split ring or annular key E projecting into an annular groove D in the casing. Accordingly the pole pieces and associated parts can be turned about the axis of the diaphragm so as to bring their length into line with the flux, to obtain the maximum volume of sound, or alternatively can be turned away from this position, thereby providing a simple form of volume control.

The pole pieces are shown as projecting somewhat beyond the casing, an arrangement that gives added sensitivity. If however such sensitivity is not required the pole pieces may terminate Within the casing.

It will be observed that the permanent magnet is in etfect shunted across the pole pieces so that if it offered a path of low reluctance to varying flux it would impair the sensitivity of the arrangement. In the arrangement in question the material of the permanent magnet is chosen so that the incremental permeability, i. e. the permeability for varying flux, is low and hence variations of flux pass almost entirely through the armature. In an alternative arrangement the permanent magnet does not connect the two pole pieces together, for example there may be two permanent magnets one lying alongside each pole piece, their circuit being completed by leakage flux from their outer ends.

In the arrangement shown in Figures 2 and 3 the headphones have no coils of any sort. In the arrangement shown in Figure 5 on the other hand the flux picked up directly by the pole pieces is reinforced by additional flux induced by a pair of coils K. These coils are connected to a larger coil L wound upon a core H which is also exposed to the fiux produced by the coil B, for example it may be an extension of one of the pole pieces H. Conveniently the earpiece is embodied in a lorgnette type of receiver, the coil L being accommodated in a handle M.

It will be appreciated that the additional coil L need not be would on a continuation of one of the pole pieces but may be independent, for example it may be embodied in the headband of a pair of headphones or may be arranged to be attached to the wearers clothing. Moreover in some cases the flux for operating the armature may be produced entirely by the coils K, no reliance being placed on flux picked up directly by the pole pieces. In general, however, a considerable proportion of the energy picked up will be dissipated by the resistance of the coils or otherwise lost in the conversion from magnetic flux to electric current and back again, and it is therefore preferred to rely as far as possible on flux picked up directly by the pole pieces H.

The coil B is shown in Figure 1 as comprising simply two rectangular turns of wire, but it will be appreciated that various arrangements may be employed in accordance with requirements, for example the number of turns may be varied, the coils may be of figure-of-eight or other multi-loop form, or a number of coils may be connected in series or parallel.

Where the coil can be at a height above the floor, for example where it is attached to the surrounding walls, it is preferred that it should be approximately at the height of the receivers, say from four to five feet from the floor. In the case of a coil of a number of turns the central turn may be at this height, other turns being higher and lower. In this case it is preferred that the latter turns should be spaced a considerable distance above and below the central turn.

In the first place this tends to equalise the intensity of the effect obtained at various positions in the space, since at a point close to the conductors the effect of the central turn will be a maximum but on the other hand the fluxes due to the turns above and below will be inclined at a substantial angle to the vertical and their horizontal components will tend to balance out. On the other hand in the middle of the space the effects of all the turns will be less intense but they will all be approximately vertical and will all reinforce one another.

In addition it is desirable to space the turns apart in order to reduce their mutual inductance and hence the total inductance of the whole coil. Thus the output of the amplifier has to work into a load of which the impedance is largely inductive, since any resistive component, due to resistance in the wire of the coil, represents wasted energy.

The output of the amplifier must as far as possible be matched to the impedance of the coil at all frequencies it is desired to reproduce faithfully, say up to 3000 or 5000 cycles per sec., and since this impedance will be at all events partially inductive it is in general preferred that the output stage of the amplifier should embody triodes or tetrodes with negative feedback. The frequency output characteristic required of the amplifier will depend on the frequency response of the earpiece, but in general a rising characteristic will be desirable.

It may however be found desirable to increase the resistance of the coil or to employ a correcting resistance in series with it as indicated at N in Figure 1. If the response curve of the receiver unit is a rising one with frequency the amount of correcting resistance will be reduced.

In one specific arrangement which has been found satisfactory an area of 1000 square feet has been found to be satisfactorily covered by a single loop comprising two turns of wire fed from an amplifier of 10 watts output. The coils were of 3/ .029" wire, and the amplifier output stage comprised two 6V6 output valves connected in ABl push-pull with anode load of 10,000 ohms, and an output transformer ratio of 26 to 1, the impedance of the loop being 15 ohms.

Taking the above as a basis the requirements for spaces of other sizes can readily be estimated by suitable modification or duplication.

While the invention is particularly applicable to systems using earphones, either single or in pairs, it can also be employed with loudspeakers, for example attached to seating in a hall or the like, in which case the complete unit will generally be substantially as already described but of considerably larger size.

Again whilst the means for producing the flux in the space has been described as a large air-cored coil it may in some cases be desirable to employ a core either instead of or in addition to an extended coil. For example in some cases it may be convenient to use a small concentrated coil and rely on a long core to distribute the flux through the space. Again where there is in any case magnetic material in the building, for example steel girders or pillars, these may be taken into account in the design of the scheme, provided the frequency response can be maintained within the desired limits.

What I claim as my invention and desire to secure by Letters Patent is:

1. A sound reproducing system for enabling sound to be reproduced at any point in a predetermined space and comprising a sound reproducing amplifier of predetermined wattage ratio and audio frequency output range, a coil installed adjacent the said space connected to and energised from the said amplifier to produce a varying magnetic field throughout the said space, and at least one receiver unit situated in the said space including a magnetic core wherein a varying magnetic flux sufficient to operate said unit is produced directly by the said varying magnetic field, an armature responsive to the flux in the core, and a sound reproducing diaphragm actuated by the said armature.

2. A system as claimed in claim 1, in which the coil is arranged to produce flux having a substantial vertical component and the receiver is oriented to respond to such a component.

3. A system as claimed. in'claim: 1: in which the: receiver core is adjustable about'the axis of the diaphragm..relatively to other parts of the receiver to serve'as avolume control.

4. A sound reproducing system for enabling sound. to be reproduced at any point in a predeterminedlspace: and comprising a sound reproducing amplifier of predetermined wattage ratio and audio frequency output range, a coil installed adjacent the said space connected to and energised from the said amplifier to produce a varying magnetic field throughout the said space, and at least one portable receiver unit freely movable by the user within the said space and having no electrical connection to other parts of the system, each said unitincluding a magnetic core wherein a varying magnetic flux sufiicient to operate said unit is produced solely bythe said varying magnetic field, an armature responsive to the flux in the core, and a sound reproducing diaphragm actuated by the said armature.

5'. A sound reproducing system for'enablingsound' to be reproduced at any point in a predetermined space and comprising a sound reproducing amplifier of' predetermined wattage ratio and audio frequency output range, a coil installed adjacent the said space connected to and energised from the said amplifier to produce a varying magnetic field throughout the said space, at least one portable receiver unit freely movable by the user within the said space and havingno electrical connection: to other parts of the system, each such unit including a: magnetic core wherein a varying magneticfiux sufficient to operate said unit is produced solely by thesaid varying" magnetic field, an armature responsive to the flux in the core, a sound reproducing diaphragm actuated by the said armature, and aheadband for fitting'to'the headof the user to support the diaphragm adjacent the users ear.

6; A system asclaimed in'claim15' in which: thecoil is arranged to produce flux with a substantial vertical component, and the receiver core is adjustable about the axis of the diaphragm relatively to the headband to serve as a volume control.

7. A sound reproducing system for enabling sound to be reproduced at any point in a predetermined space and comprising a sound reproducing amplifier of predetermined wattage ratio and audio frequency output range, a coil installed adjacent the said space connected to and energised from the said amplifier to produce a varying magnetic field throughout the said space, and at least one receiver unit situated in the said space including a magnetic core wherein a varying magnetic flux suflicient to operate said unit is produced directly by the said varying magnetic field, an armature responsive to the flux in the core, and a sound reproducing diaphragm actuated by the said armature, the said core comprising a pair of pole pieces of high permeability extending in opposite directions from the armature whereby the latter provides a path for magnetic flux between their adjacent ends via small air gaps, and at least one permanent magnet arranged to produce a permanent flux through the air gaps.

8. A system as claimed in claim 7 in which the receiver has a casing and the end of at least one of the pole pieces projects from the casing.

9. A system as claimed in claim 7 in which the receiver has a casing with at least one protuberance in which the end of one of the pole pieces is accommodated.

10. A system as claimed in claim 7 in which the permanent magnet is of high retentivity but low incremental permeability and is connected so as to bridge the pole pieces.

11. A sound reproducing system for enabling sound to be reproduced at any point in a predetermined space and comprising a sound reproducing amplifier of predetermined wattage ratio and audio frequency output range, a coil installed adjacent the said space connected to and energised from the said amplifier to produce a varying 6. v a magnetic held throughout the said space, and atleastorie portable receiver. unit freely movable by the: user within the said space and having no electrical connectiontoother parts of the system, each said unit including'a magnetic core wherein a varying; magnetic flux sufiicient to operate said unit is produced solely by the said varying magnetic field, an armature responsive to the fluxin the core, and a sound reproducing diaphragm actuated by the said armature, the said core comprising a pair of pole pieces of high permeability extendingin oppositedirections from the armature whereby the latter provdes a path for magnetic flux between their adjacent ends via small air gaps, and at least one permanent magnet arranged to produce a permanent flux through the air gaps.

12. A system as claimed in claim 11 in which the receiver has a casing and the endof at least one of thepole pieces projects from the casing;

13. A system as claimed in claim- 11 in which the receiver has a casing with at least one protuberance in which the end' of one of the pole pieces is accommodated.

14. A system as claimed in claim 11 inwhich. the permanent magnetis of'high retentivity but low incremental permeability and is connected so as to bridge the pole pieces.

15..A sound reproducing system for enabling sound to be reproduced at anypoint'in a predetermined space and comprising a sound reproducing amplifier of. predetermined wattage ratio' and audio frequency output range, a coil installed adjacent the said space connected to and energised from the saidamplifier to produce a varying magnetic field throughout the said space, and at. least one receiver unit situated in the said space and having a magnetic core whereina varying magnetic flux sufficient to operate. said unit is. produced solely bythe said varying magnetic field, an armature responsive to the flux in the core, and a sound reproducing diaphragm actuated by the said armature.

16. A sound reproducing system for enabling sound to be reproduced at any point in a predetermined space and comprising a sound reproducing amplifier of predetermined wattage ratio and audio frequency output range, a coil installed adjacent the said space connected to and energised from the said amplifier to produce a varying magnetic field throughout the said space, and at least one receiver unit situated in the said space including a magnetic core wherein a varying magnetic flux sufficient to operate said unit is produced directly by the said varying magnetic field, an armature responsive to the flux in the core, a sound reproducing diaphragm actuated by the said armature, an energising coil associated with the said core to energise it, a second core wherein a varying magnetic flux is produced by the said varying magnetic field, and a search coil linking the said second core and connected to the energising coil to produce in the first core additional flux reinforcing that picked up directly from the varying magnetic field.

17. A system as claimed in claim 16 in which the second core is a continuation of the first.

18. A system as claimed in claim 16 in which the receiver has a handle and the second core and search coil are accommodated in the said handle.

19. A sound reproducing receiver unit responsive to an audio frequency varying magnetic field in the surrounding space having a magnetic core wherein a varying magnetic flux is produced directly by said varying magnetic field, a permanent magnet associated with said core to maintain a bias magnetic flux therein, an armature responsive to the resultant magnetic flux in said core, and a sound reproducing diaphragm actuated by said armature.

20. A portable sound reproducing receiver unit responsive to an audio frequency varying magnetic field in the surrounding space having a substantially vertical component, said unit consisting of a magnetic core wherein a varying magnetic flux is produced directly by said varying magnetic field, an armature responsive to the flux in the core and a sound reproducing diaphragm actuated by said armature, and means supporting said diaphragm and core with their respective axes in cooperative relation with said magnetic field.

21. A unit as claimed in claim 20 in which the receiver is provided with means for adjustably supporting the core for movement about the axis of the diaphragm relatively to other parts of the receiver to serve as a volume control.

22. A unit as claimed in claim 21 in which the receiver is provided with a headband for attachment to a listeners head to support the sound reproducing diaphragm in comfortable listening position, while permitting adjustment of the receiver core with respect to the axis of the diaphragm.

23. As an article of manufacture a sound reproducing receiver unit responsive to an audio frequency varying magnetic field in the surrounding space and including a magnetic core means disposed so that a varying magnetic flux is produced in it by the said varying magnetic field, an armature responsive to the flux in the core and a sound reproducing diaphragm actuated by the said armature, the said core comprising a pair of pole pieces of high permeability extending in opposite directions from the armature and separated therefrom by small air gaps whereby said armature provides a path for magnetic flux between the adjacent ends of said pole pieces, and at least one permanent magnet constituting means to produce a permanent flux through said armature and air gaps, said magnetic core means providing a substitute for any electrical connection from the receiver unit to an outside power source.

24. As an article of manufacture, a portable sound reproducing receiver unit responsive solely to an audiofrequency varying magnetic field in the surrounding space and including a magnetic core disposed so that a varying magnetic flux is directly produced in it by the said varying magnetic field, an armature responsive to the fiux in the core and a sound reproducing diaphragm actuated by the said armature, the said core comprising a pair of pole pieces of high permeability extending in opposite directions from the armature and separated therefrom at their adjacent ends by small air gaps, and at least one permanent magnet arranged to produce a permanent flux through the air gaps, the pole pieces being arranged to protrude outwardly beyond the outer limits of the permanent magnet.

25. A receiver unit as claimed in claim 24, including an outer casing enclosing all the parts thereof except the pole pieces which protrude outwardly through and beyond the outer surface of the casing.

26. As an article of manufacture, a sound reproducing receiver unit responsive to an audio frequency varying magnetic field in the surrounding space, which consists om a magnetic core of high permeability wherein a varying magnetic flux is produced directly by the said varying magnetic field, a permanent magnet of low incremental permeability producing a constant flux in said core, an armature responsive to the flux in the core, a sound reproducing diaphragm actuated by the said armature, and a surrounding casing.

References Cited in the file of this patent UNITED STATES PATENTS 1,503,529 Finley a- Aug. 5, 1924 1,583,581 Dominquez May 4, 1926 1,830,160 French et al. Nov. 3, 1931 2,252,641 Poliakoff et al Aug. 12, 1941 2,567,431 Halstead Sept. 11, 1951 FOREIGN PATENTS 495,171 Great Britain Nov. 8, 1938 592,990 Great Britain Oct. 6, 1947 650,954 France Aug. 22, 1927 

